The tragic news that forces linked to al-Qaeda have retaken Fallujah is just the latest reminder that George W. Bush's war of choice was a historic, catastrophic misjudgment. "Any Republican seeking nomination for the 2016 presidential election should at a minimum be willing to admit Iraq was a mistake," Jeremy Lott writes at RealClearWorld. "It was an error that cost us upwards of $1.5 trillion, thousands of U.S. lives and hundreds of thousands of Iraqi dead, while seriously hindering our efforts to track down the real culprits of September 11, 2001."

The issue is going to be tough for Republicans to navigate, given that public opinion has turned against the war, even as powerful GOP factions still support it. Marco Rubio and Chris Christie are probably already gaming out their strategy. What's less remarked upon is the challenge Iraq will pose for Democrats. The war was foisted on America by a propagandizing Republican administration, and Democratic hawks have been better than Republican hawks at acknowledging error. But as Daniel Larison notes, "Virtually none of the politicians mentioned as 2016 candidates in the GOP were even in national office during the Bush year," whereas several prominent Democrats, including Hillary Clinton, Joe Biden, and John Kerry, all have pro-war votes on their resumes. (And it's hard to believe that any of those three were hoodwinked by Dick Cheney.)

An irony of Obama's presidency is his decision to elevate so many Iraq War supporters after arguing that his own opposition to the war showed his superior foreign-policy judgment. Those staffing decisions have consequences for the Democratic bench.

An irony of Obama's presidency is his decision to elevate Iraq War supporters after arguing that his opposition to the war showed his superior foreign-policy judgment.

It's conceivable that someone like Rand Paul will win the GOP nomination, and the general election will feature a Republican nominee attacking his Democratic opponent's war support in much the same way that Obama successfully attacked Clinton: "You thought the Iraq War was a prudent invasion, and now you want to be president?"

If urging the most catastrophic foreign-policy mistake in a generation isn't disqualifying, what is?

Writing about the GOP, Larison argues that it is not enough for the nominee to mouth reassurances about Iraq to skeptical voters. The next Republican president "must show that he understands why the Iraq war was a mistake and knows how to avoid making similar foreign policy errors. If Republican candidates accept that invading Iraq was a mistake, but still think launching an attack on Iran is acceptable or even preferable, that suggests they haven’t really learned anything. In addition to admitting that the war was a mistake, Republicans ... need to be able to articulate why preventive war ... is neither wise nor prudent."

I'd make that a bipartisan litmus test.

There is next to no chance I'd vote for a Republican who thought the Iraq War was prudent, or showed an eagerness for more military interventions. But neither is it enough for a Democratic candidate to claim that the problem with the Iraq was the way that the Bush Administration executed it. Habitual hawks like Clinton have to clear a particularly high hurdle. And no one should be elected president without showing, beyond any doubt, that they understand why the war was a mistake and how to avoid like mistakes in the future.

Beyond understanding Iraq's lessons, what foreign policy vision would I like to hear? Something like what Andrew Bacevich sketches in a recent Los Angeles Times op-ed (emphasis added):

Force is good for some things, preeminently for defending what is already yours. Not content to defend, however, the United States in recent decades has sought to use force to extend its influence, control and values. In a world divided between haves and have-nots, between postmodern and pre-modern, and between those for whom God is dead and those for whom God remains omnipresent, expecting coercion to produce reconciliation, acceptance or submission represents the height of folly. So force employed by the United States in faraway places serves mostly to inflame further resistance, a statement that is true whether we're talking about putting "boots on the ground" or raining down Hellfire missiles from the heavens.

What then is to be done? That which Washington is least capable of undertaking: Those charged with formulating policy must think anew. For starters, that means lowering expectations regarding the political effectiveness of war, which is demonstrably limited. Take force off the metaphorical table to which policymakers regularly refer. Rather than categorizing violence as a preferred option, revive the tradition of treating it as a last resort. Then get serious about evaluating the potential for employing alternative forms of power, chiefly economic and cultural, to advance American interests.

But it's hard to imagine any presidential candidate from either party saying all of that.

Conor Friedersdorf is a staff writer at The Atlantic, where he focuses on politics and national affairs. He lives in Venice, California, and is the founding editor of The Best of Journalism, a newsletter devoted to exceptional nonfiction.